Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle body structure has generated interest in joining steel workpieces to aluminum-based (aluminum or aluminum alloy) workpieces by resistance spot welding. In particular, the ability to resistance spot weld workpiece stack-ups containing different workpiece combinations (e.g., steel/steel, aluminum-based/steel, and aluminum-based/aluminum-based) with one piece of equipment would promote production flexibility and reduce manufacturing costs.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through overlapping metal workpieces and across their faying interface to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically clamped at diametrically aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface. When the metal workpieces being spot welded together are a steel workpiece and an aluminum-based workpiece, the heat generated at the faying interface initiates a molten weld pool extending into the aluminum-based workpiece from the faying interface. This molten weld pool wets the adjacent surface of the steel workpiece and, upon cessation of the current flow, solidifies into a weld nugget that forms all or part of a weld joint.
In practice, however, spot welding a steel workpiece to an aluminum-based workpiece is challenging since a number of characteristics of those two metals can adversely affect the strength—most notably the peel strength—of the weld joint. For one, the aluminum-based workpiece usually contains one or more refractory oxide layers on its surface. The oxide layer(s) are typically composed of aluminum oxides, although other oxide compounds may also be present. For example, in the case of magnesium-containing aluminum alloys, the oxide layer(s) also typically include magnesium oxides. The oxide layer(s) present on the aluminum-based workpiece are electrically insulating and mechanically tough. As a result of these physical properties, the oxide layer(s) have a tendency to remain intact at the faying interface where they can hinder the ability of the molten weld pool to wet the steel workpiece. Efforts have been made in the past to remove the oxide layer(s) from the aluminum-based workpiece prior to spot welding. Such removal practices can be unpractical, though, since the oxide layer(s) have the ability to self-heal or regenerate in the presence of oxygen, especially with the application of heat from spot welding operations.
The steel workpiece and the aluminum-based workpiece also possess different properties that tend to complicate the spot welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and relatively high electrical and thermal resistivities, while the aluminum-based material has a relatively low melting point (˜600° C.) and relatively low electrical and thermal resistivities. As a result of these physical differences, most of the heat is generated in the steel workpiece during current flow. This heat imbalance sets up a temperature gradient between the steel workpiece (higher temperature) and the aluminum-based workpiece (lower temperature) that initiates rapid melting of the aluminum-based workpiece. The combination of the temperature gradient created during current flow and the high thermal conductivity of the aluminum-based workpiece means that, immediately after the electrical current ceases, a situation occurs where heat is not disseminated symmetrically from the weld site. Instead, heat is conducted from the hotter steel workpiece through the aluminum-based workpiece towards the welding electrode in contact with the aluminum-based workpiece, which creates a steep thermal gradient between the steel workpiece and the welding electrode.
The development of a steep thermal gradient between the steel workpiece and the welding electrode in contact with the aluminum-based workpiece is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum-based workpiece after the electrical current has ceased, the molten weld pool solidifies directionally, starting from the region nearest the colder welding electrode (often water cooled) associated with the aluminum-based workpiece and propagating towards the faying interface. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, micro-cracking, and surface oxide residue—towards and along the faying interface within the weld nugget. Second, the sustained elevated temperature in the steel workpiece promotes the growth of brittle Fe—Al intermetallic compounds at and along the faying interface. The intermetallic compounds tend to form thin reaction layers between the weld nugget and the steel workpiece. These intermetallic layers are generally considered part of the weld joint, if present, in addition to the weld nugget. Having a dispersion of weld nugget defects together with excessive growth of Fe—Al intermetallic compounds along the faying interface tends to reduce the peel strength of the final weld joint.
In light of the aforementioned challenges, previous efforts to spot weld a steel workpiece and an aluminum-based workpiece have employed a weld schedule that specifies higher currents, longer weld times, or both (as compared to spot welding steel-to-steel), in order to try and obtain a reasonable weld bond area. Such efforts have been largely unsuccessful in a manufacturing setting and have a tendency to damage the welding electrodes. Given that previous spot welding efforts have not been particularly successful, mechanical processes such as self-piercing rivets and flow-drill screws have predominantly been used instead. Both self-piercing rivets and flow-drill screws are considerably slower and have high consumable costs as compared to spot welding. They also add weight to the vehicle body structure which at some point can begin to counteract the weight savings attained through the use of aluminum-based workpieces in the first place. Advancements in spot welding that would make the process more capable of joining steel and aluminum-based workpieces would thus be a welcome addition to the art.